Functional Groups
A discussion about molecules, especially those within organic chemistry, isn’t complete without a primer on functional groups. Why though? Functional groups have a unique set of defining characteristics that define both the properties and reactions molecules can undergo. Ultimately they decide what are molecule can and can’t do in addition to the name that molecule will take.
A Bunch of Carbons
First, up we will take a look at the alks, which are our most basic molecules. They contain carbons bonded to hydrogens and other carbons. The defining factor between the different functional groups is whether or not they have a single, double, or triple bond.
Overall the alks are nonpolar so the only intermolecular force they experience are van der Waals forces (VDW). As a result, they have weak intermolecular forces and consequently low melting and boiling points.
Alkane
Alkanes contain nothing, but single bonds. They really are the most vanilla of all molecules and hardly if ever undergo reaction of any type.
In terms of naming alkanes have the suffix -ane. So a six carbon molecule with single bonds would be called hexane.
Alkene
While alkenes contain at least one double bond. They are in many ways are identical to alkanes, however, their double bond is slightly more reactive so they will undergo addition reactions at the double bond.
In terms of naming alkenes have the suffix -ene. So a six carbon molecule with a double bond would be called hexene.
Alkyne
Last, but not least an alkyne contains at least one triple bond. Again the triple bond like the double of an alkene is reactive and undergoes double addition reactions.
When naming an alkyne the suffix is -yne so our same six carbon molecule would be called hexyne.
Alkyl Halide
All the Oxygens
Up next for our bigger categories, the oxygen-containing functional groups. Since oxygen is more electronegative than carbon and hydrogen these molecules tend to be polar. Additionally, any functional groups with OH bonds can hydrogen bond. This means that the oxygen containing functional groups have substantially stronger intermolecular forces than the alkanes and consequently higher melting and boiling points.
Ether
Ethers consist of an oxygen atom connected to two carbon groups on either side of it. Typically the MCAT will discuss symmetrical ethers in which the carbon chains on either side of the oxygen are the same, although asymmetrical ethers do exist.
While they do have polar bonds they are so weakly polar that they are considered nonpolar. This means that compared to many other functional groups in this section they exist as gases due to their low melting and boiling points. Ethers are pretty unreactive but are slightly more so than alkanes due to their slight dipole.
Naming for an ether is a bit different than other molecules since the six carbons in our ether will be broken in half and attached on either side of the oxygen atom. This means that instead of taking the hex prefix our molecule will be dipropyl with the ether’s suffix of, well, ether. So our molecule will be called dipropyl ether.
Epoxide
Epoxides are simply a special kind of ether, specifically a cyclic ether. Since an epoxide has significant ring strain due to its overall structure making them far more reactive than other ethers. Otherwise, they possess the same properties as other ethers.
Alcohol
The alcohol functional group contains either single or multiple OH bonds. As a result, alcohols have the ability to hydrogen bond making their intermolecular forces particularly strong. They have high melting and boiling points due to this and tend to exist as liquids.
This also means that they are decently reactive since the OH can acts as a leaving group albeit a fairly weak. Additionally, they undergo several other reactions that we will explore in more detail later.
Unlike ethers they revert to the normal naming scheme we have seen with the alks. So a six carbon alcohol would be called hexanol since alcohols have a suffix of -ol.
Aldehyde
Aldehydes are molecules with a terminal carbon double bonded to an oxygen. They are distinguished from ketones which we will look at next because the carbonyl, the carbon double bonded to the oxygen, is on one of the interior carbons.
Both molecules have nearly identical properties which include dipole-dipole interactions, moderate boiling and melting points, and reactivity at the carbonyl. Generally speaking, the carbonyl oxygen draws electrons away from the carbon to which it is attached making it slightly positive and the oxygen slightly negative. Make sure to look out for carbonyls as you go as reactions frequently occur there.
In terms of naming they have the suffix -al and a six carbon aldehyde would therefore be called hexanal.
Acetal and Hemiacetal
Acetals and hemiacetals are derivatives of aldehydes